Abstract
We present new mineral and whole-rock compositions and Sr-Nd-Hf-Pb-O-H isotope data on samples from the Kangerlussuaq Alkaline Complex (similar to 1000 km(2)) in central East Greenland, part of the North Atlantic Igneous Province. This complex mainly consists of the Kangerlussuaq Intrusion but includes at least 13 separate satellite intrusions emplaced in the uppermost crust close to the unconformity between Archaean gneisses and overlying Palaeogene flood basalts. The complex is divided into (i) older satellite intrusions (similar to 55-53 Ma) composed of multiple syenites and granites and minor gabbros and peridotites, (ii) the voluminous Kangerlussuaq Intrusion (similar to 50 Ma), which displays a gradual transition from quartz syenites (nordmarkites) at the margin to nepheline syenites (foyaites) in the centre, and (iii) younger satellite intrusions (similar to 47-45 Ma) of minor syenites, granites and diorites concentrated southeast of the Kangerlussuaq Intrusion. The complex displays a temporal evolution in which SiO2 decreases (74-56 wt.%) and total alkalis (6-16 wt.%), amphibole Na+K content, Pb-206/Pb-204(meas), epsilon(Ndi) and epsilon(Hfi) (+3 to +11) increase from the older intrusions through the nepheline syenites. This is followed by a reversal to higher silica (62-73 wt.%) and lower total alkalis (9-12 wt.%), amphibole Na+K content, Pb-206/Pb-204(meas), epsilon(Ndi) and epsilon(Hfi) (-13 to +2) in the younger satellite intrusions.
Temporal changes in the location of magma plumbing systems and in magma production rates played a profound role in controlling silica content, alkalinity and degree of crustal contamination during development of the complex. Phonolitic magma was only generated after prolonged magmatism had shielded the conduits from interaction with country rock. The parental magmas were probably basanitic to alkali olivine basaltic in composition. The older satellite intrusions and the Kangerlussuaq Intrusion have low delta O-18 magma values (-1 to +6 parts per thousand) compared to the younger, more crustally contaminated, satellite intrusions (+4.5 to +7 parts per thousand). It appears that the magmas only had sufficient over-pressure to intrude the basalt cover when larger volumes of less contaminated trachyte magma were produced, resulting in the generation Of low-delta O-18 magmas due to dehydration of hydrothermally altered basalt xenoliths. (C) 2006 Elsevier B.V. All rights reserved.